Separation of gamma benzene hexachloride by distillation



United States Patent -No Drawing. A cation-0rtober3fl, 1352, lit.v N0- 317,815.

2 Claims. Cl. 202-57 T hi invention relates to the treatment: of enzene hcxachloride ,2,.3u4.15. -hexachlorocyc1qhexam:2 and. more.

particularly to a method .fortthe. separation-ofthe. various isomers thereof.

It is known that benzene hexachloride (BHCLoW -MQ in a number of isomeric forms. For example, inJhe. addition chlorina ion of b mzeua; at. least. five su h i omers are; formed. having approximately: therfollowing weight 1 is ribution: lpha, 65 p ce t & p r ent; gamma, in r en delta, 10 P rc nt; eps n,,-. less h n. 1; per

ent; an he. emain er, hept chlorocyclqh xanes and. 1

other impurities. It is also known that the gamma isomer ot.-B.H ifsy. far the most inseetieidally active..isemer.

Accor ingly. i is frequently. desired-to concentrate mixtures with respect-to. the gamm isomer to fa ilita the preparation of irnprovedinsecticid l. .torrnulations by. the elimination of the inert isomersand. odonfer us constitu nts thereof. .In ddi on, someottheptherBHC isomer such a he. delta i omer, show. promise fomo her a d, similar commercial; purposes.

. At. thepresent. time. he. gamm -isomer islrecovered, from technical or crude BHC isomer mixtures byg-ex traction and fractional. crystallization using selected solvents. While quite satisfactory for-effecting separation of the relatively soluble gamma isomer from the relatively insoluble alpha isomer, such a procedure is difficult when attempting to separate the gamma isomer from the highly soluble delta isomer. In consequence, a large-quantity (up to about per cent of the gamma isomer) is frequently lost in a high delta isomer stream which cannot subsequently be used in the manufacture of lindane (above 99 per cent gamma BHC).

It is accordingly an object of this invention to provide an improved method for the treatment of BHC and more particularly for the separation of the various isomers thereof. Another object is to provide a method adapted to the separation of the gamma and delta BHC isomers, and particularly a method which can be employed to recover the gamma isomer from high delta isomer streams obtained in prior fractional extraction and crystallization processes.

It has now been found that the various benzene hexachloride isomers can be separated by distillation of BHC isomer mixtures with the aid of a selected group of solvents. Upon distillation, the various BHC isomers can be recovered as an overhead or bottoms in a plurality of fractions or cuts. According to this process, the gamma isomer is recovered in exceedingly high yields as an overhead fraction, whereas the delta isomer remains almost completely in the bottoms fraction. In addition, further separation of the gamma isomer from other isomers can be accomplished by recovering the overhead in a number of distinct cuts, some of which are richer in the gamma isomer than in other isomers.

In practice, the number of cuts are determined by the degree of isomer separation desired. If, for example, it is merely desired to separate the gamma and delta isomers, the entire overhead can be recovered as the high uct substantially'all'ofthe gamma isomer.

2,7 l 7 ,233 5 Haunted Seph- 6, 19.55

gamma fraction, and the bottoms recovered as the high deltafraction. However, if it is desired to separate the gamma, delta and alpha isomers, the latter isomer is recovered-as the first('most volatile) overhead fraction, the gamma as the second overhead fraction, and the delta isomer is recovered either as athird overhead fraction or preferably as the bottoms of thedistillation. .More com? plete isome Separations can be, accomplished by increasingthe number of overhead cuts.

This process can be carried out continuously, if deredt by stripp ng off any desir d. raction, sushi BSrbY th use-ofa-p ra i yofistillati n c mn Thus, a wo. column ys em can be: employ d t remove a alpha isomer an hep a h oro-cy lohexane m x ure n he-fir t o umn.- nd; a h gh gamma iscmerwfrac ion inthe se a column. The bottoms from the second column, haying.

a high delta isomer content, canthcn be recovered as such, or can be further distilled in a third column to additionally purify the The; solvents-useful. with thepresent invention are of a polar nature and have a boiling: point between about; 2&0 and 300 C. Typical examplesof such solvents are highly polar compounds having a plurality of hydroxyl groups, such as triethylene glycol (boiling point of 289 C.) and glycerine (boiling point of 291 C.).

The: quantity of solvent employed per unit weight of BHC treated is not critical. However, the overhead of the distillation has a relatively definite proportion of BHC solvent, and, accordingly,.it is generally preferred to employ sulfiicient solvent to -remove-substantially.allof the isomers it ,is desired to separate. 'fhus in recovering the gamma: isomer 'tfrom delta isomer-containing. BHC mixtures, theminimum quantity of solvent to be employed is that which will at least distill over as an overhead prod- Additional quantities of solvent can be present in the mixture :being distilled without adversely affecting the results ofthe separation, provided the high-delta isomer fraction is recovered separately f-rom thehigh gamma isomer fraction or fractions. 4

The temperature of the distillation is determined by the mixture being distilled, i. e., the boiling point of the solvent, the concentration and isomer distribution of the benzene hexachloride dissolved therein, and the positive pressure of the distillation system.

The distillation is preferably carried out at reduced pressure, generally below about 25 mm. and preferably below about 5 mm. absolute pressure. At such pressures, the distillation can be conducted at reduced temperatures and thereby minimize the possibility of cracking or dehydrochlorination of the BHC.

The number of plates used in the distillation column or columns is not critical to the successful operation of this process. In general, the greater the number of plates employed, the more complete the separation. Generally, between 5 and 15 plates has been found adequate for most separations. improve the separation of the isomers.

The following examples illustrate the features of the present invention:

EXAMPLE I A benzene hexachloride mixture parts by weight) containing 32 per cent gamma isomer, 12 per cent alpha isomer, 35 per cent delta isomer, and the remainder, a mixture of beta and epsilon isomers containing some heptachlorocyclohexane was distilled with 750 parts by weight of triethylene glycol. The overhead temperature of the distillation covered a range of between and 152 C. Most of the BHC distilled over at a temperature between and C. The pressure of the distillation was maintained at about 5 mm. absolute pres- A reflux can also be employed to v sure. The isomer distribution in the various cuis are given in Table I.

Table I.(Weight per cent) BHC i e ta C OIO' Cut No. cyclm Alpha Delta Epsilon Gamma hexane 13. 5 e. 9 4s. 9- 4. 7 26. 3 33. 8 2. 3 58. 0 23. -l' 2. 9 7B. 4 17. 9 6. 6 66. 4 16. 9 14. 1 32. 7 28. 4

In this test, 88 per cent of the original gamma isomerwas obtained as a 53 per cent gamma isomer product At the same time, only about 3 per cent of the original quantity of delta isomer was obtained in the overhead fraction, and the final product merely contained about 2 per cent delta isomer.

EXAMPLE II 7 Example I was repeated except that the overhead fraction was separated into 7 cuts. The composition of the various cuts is given in Table No. II.

Table II.-(Wez'ght per cent) In considering only cuts 2-6 inclusive, 92 per cent of the original gamma isomer was recovered as a 61 per. cent gamma isomer product. At the same time, only 5 per cent of the original delta isomer was obtained in these cuts and the product only contained per cent delta isomer.

4 EXAMPLE HI Example I is repeated except that glycerine is substi= tuted for triethylene glycol. The overhead product obtained from this distillation contains a BHC isomer distribution similar to that of Example 1. However, using glycerine, somewhat greater ratios of BHCzsolvent were obtained than when using triethylene glycol.

EXAMPLE 1v Example Ii is repeated except that glycerine is substituted for triethylene glycol as the solvent. sults are obtained.

As is believed apparent from the foregoing, the present invention provides an improved method for the separation of BHC isomers, and particularly for the separation of the difficultly separable gamma and delta isomers. This invention thus provides a convenient and effective Similar remeans for separating'a wide variety of BHC isomer mixtures, and one which is particularly adapted to the recovery of the gamma isomer from the high delta streams obtained in prior fractional extraction and crystallization methods. 7

I claim:

l. A method for treating benzene hexachloride isomer mixtures comprising distilling the isomer mixture in the presence of a polar solvent selected from the group consisting of triethylene glycol and glycerine, and recoveringthe benzene hexachloride in a' plurality of fractions from said distillation. 2; A method for treating benzene hexachloride isomer mixture containing the gamma and delta isomers com prising distilling the isomer mixture in the presence of a polar solvent selected from the group consisting of triethylene glycol and glycerine until a major portion of the gamma isomer is distilled from the mixture and interrupting said distillation whilea major portion of the delta isomer still remains undistilled, and recovering the gamma isomer fraction separate from the delta isomer fraction. i

No references cited. 

2. A METHOD FOR TREATING BENZENE HEXACHLORIDE ISOMER MIXTURE CONTAINING THE GAMMA AND DELTA ISOMERS COMPRISING DISTILLING THE ISOMER MIXTURE IN THE PRESENCE OF A POLAR SOLVENT SELECTED FROM THE GROUP CONSISTING OF TRIETHYLENE GLYCOL AND GLYCERINE UNTIL A MAJOR PORTION OF THE GAMMA ISOMER IS DISTILLED FROM THE MIXTURE AND INTERRUPTING SAID DISTILLATION WHILE A MAJOR PORTION OF THE DELTA ISOMER STILL REMAINS UNDISTILLED, AND RECOVERING THE GAMMA ISOMER FRACTION SEPARATE FROM THE DELTA ISOMER FRACTION. 